1. Field of the Invention
The present invention relates to a liquid crystal display device widely incorporated in a liquid crystal panel for OA (Office Automation) apparatus, AV (Audio Vidual) apparatus or the like, and a method for producing the liquid crystal display device.
2. Description of the Related Art
Presently, various display modes have been found for liquid crystal display devices. Particularly, liquid crystal display devices having TN (Twisted Nematic) display modes, STN (Super Twisted Nematic) display modes, or the like, are mass-produced.
Usually, a liquid crystal material used for realizing any one of the above-mentioned modes is used with a chiral agent which adjusts a helical pitch of the liquid crystal material. The helical pitch of the liquid crystal material can be controlled and determined whether to increase or decrease upon a rise in the temperature in accordance with the kind of the chiral agent to be used.
Presently, CN (Cholesteryl nonanoate produced by Chisso Corp.) or S-811 (produced by Merck and Co., Inc.) is generally used as the chiral agent. These chiral agents increase the helical pitch of the liquid crystal material upon rise in temperature. Generally, the helical pitch of the liquid crystal material may be either increased or decreased upon a rise in the temperature as long as a stable orientation, i.e., a stable d/p value of liquid crystal molecules can be achieved at an operating temperature of the liquid crystal display device, namely, a temperature in a range of about 0xc2x0 C. to about 40xc2x0 C. The d/p value is defined by a thickness d of a liquid crystal cell and a helical pitch p, of the liquid crystal material.
A technique is described in Japanese Laid-Open Patent Publication No. 7-258641 in which a mixture of two kinds of optically active substances is used where one increases a helical pitch of a liquid crystal material and the other decreases the helical pitch of the liquid crystal material upon a rise in the temperature. Accordingly, a helical pitch of the liquid crystal material hardly changes upon a rise in the temperature.
As described in Japanese Laid-Open Patent Publication No. 7-218915, in the case where a surface of each substrate is rubbed, the twist angle of the liquid crystal molecules is determined by a helical pitch p obtained at a nematic-isotropic phase transition temperature (hereinafter, simply referred to as xe2x80x9cTNxe2x88x92Ixe2x80x9d) (C.xc2x0) of a liquid crystal material with a chiral agent added, a cell thickness d and surface controlling force caused by a rubbing treatment. Therefore, the difference between a dip value obtained at the vicinity of TNxe2x88x92I and a d/p value obtained at a temperature where the liquid crystal display device is in operation should not be large.
Moreover, Japanese Laid-Open Patent Publication No. 6-265899 describes a method for obtaining an orientation at an interface of each substrate without performing a rubbing treatment and by utilizing a helical pitch of a liquid crystal material. According to this method, a chiral agent is used that decreases the helical pitch of the liquid crystal material upon a rise in temperature so as to restrain stripe domains from generating, thereby preventing deteriorations of a display contrast and a response rate of the display. In this case, liquid crystal molecules are oriented in a twisted manner, the orientation being determined by a d/p value at TNxe2x88x92I of the liquid crystal material with the chiral agent added. Thus, for example, in order to produce a TN type liquid crystal display device having liquid crystal molecules twisted by 90xc2x0 between a pair of substrates, the cell gap (the cell thickness) d and the helical pitch p of a liquid crystal cell is adjusted so as to satisfy the relationship, d/p=0.25 at the vicinity of the TNxe2x88x92I.
The present inventors have proposed a liquid crystal display device including a display medium having liquid crystal regions microscopically divided by polymer walls (Japanese Laid-Open Patent Publication No. 6-301015). The liquid crystal display device is produced as follows.
First, a pair of substrates, each having transparent electrodes for driving the liquid crystal display device, are provided in an opposing manner such that the transparent electrodes are located inside. Subsequently, a mixture containing at least a liquid crystal material, a chiral agent, a photopolymerizable resin and a photopolymerization initiator is interposed between the opposing pair of substrates. Then, the substrates including the mixture are heated to such a degree that the display medium inside is in an isotropic state. Thereafter, UV light having distribution of weak and strong intensities is radiated to the mixture while maintaining the temperature.
Then, the liquid crystal material and the photopolymerizable resin are phase separated by gradually cooling to room temperature with the photopolymerizable resin placed in regions radiated with light of strong intensity and the liquid crystal material placed in regions radiated with light of weak intensity, thereby producing the liquid crystal device.
In the above-described method for producing a liquid crystal display device, the liquid crystal material and the photopolymerizable resin are gradually cooled to phase separate after being irradiated with UV light. The orientation state of the liquid crystal molecules is roughly determined during the cooling step at the vicinity of TNxe2x88x92I where droplets of the liquid crystal material start to appear. A significant amount of photopolymerizable resin still remains in the liquid crystal material at the vicinity of TNxe2x88x92I. In this case, if a chiral agent is used that increases the helical pitch of the liquid crystal material upon a rise in temperature, the liquid crystal material tends to have a helical pitch longer than that of a liquid crystal material having no photopolymerizable resin remaining therein. Since the orientation of liquid crystal molecules is affected by a d/p value, in the above case, the longer helical pitch results in a smaller d/p value. As a result, the liquid crystal molecules do not orient in an intended twisted manner and often exhibit orientation with a smaller twist angle. For example, in the case of producing an STN type liquid crystal display device with a twist angle of 240xc2x0, when the d/p value is smaller than the intended d/p value due to the increased helical pitch, a so-called low twisting phenomenon is caused in which case the liquid crystal molecules are not twisted by 240xc2x0 but by 60xc2x0. Accordingly, when a chiral agent is used which increases a helical pitch of the liquid crystal material upon a rise in temperature, orientation of the liquid crystal molecules become unstable, thereby causing a problem of not being able to obtain an intended pitch.
According to one aspect of the present invention, a liquid crystal display device includes: a pair of substrates; polymer walls; and a display medium interposed between the pair of substrates, including a liquid crystal region partially provided with or completely surrounded by the polymer walls. The liquid crystal region includes a liquid crystal material having a helical pitch, the helical pitch being either unchanged or reduced upon rise in temperature.
In one embodiment of the present invention, liquid crystal molecules in the liquid crystal regions are oriented in a twisted manner.
In another embodiment of the present invention, the liquid crystal molecules have a twisted angle of about 240xc2x0.
In another embodiment of the present invention, liquid crystal molecules in the liquid crystal region are oriented in an axially-symmetric manner with respect to an axis perpendicular to the pair of substrates.
According to another aspect of the present invention, a method for producing a liquid crystal display device includes the steps of: arranging a pair of substrates to oppose each other with a predetermined gap therebetween; injecting a mixture containing at least a liquid crystal material, a photopolymerizable material and a photopolymerization initiator between the pair of substrates; irradiating the mixture with light while heating the mixture; and gradually cooling the mixture to phase-separate the liquid crystal material and the photopolymerizable material contained in the mixture. The liquid crystal material has a helical pitch, the helical pitch being either unchanged or reduced upon rise in temperature.
In one embodiment of the present invention, liquid crystal molecules in the liquid crystal material are oriented in a twisted manner.
In another embodiment of the present invention, the liquid crystal molecules have a twisted angle of about 240xc2x0.
In another embodiment of the present invention, liquid crystal molecules in the liquid crystal material are oriented in an axially-symmetric manner with respect to an axis perpendicular to the pair of substrates.
According to the present invention, a display medium provided between a pair of substrates is gradually cooled to room temperature after a light radiation in order to phase separate a liquid crystal material and a photopolymerizable resin contained in the display medium such that the display medium has liquid crystal regions made of the liquid crystal material and polymer walls surrounding the liquid crystal regions. In this case, the display medium contains a chiral agent which either reduces or does not change a helical pitch p of the liquid crystal material upon a rise in temperature. Such a chiral agent allows a d/p value to stay in a range capable of obtaining an originally intended twist angle at the vicinity of TNxe2x88x92I where the droplets of the liquid crystal material start to appear. Thus, the desired twist angle can be obtained which is roughly determined at the vicinity of TNxe2x88x92I.
Thus, the invention described herein makes possible the advantages of (1) providing a liquid crystal display device capable of achieving an intended helical pitch of a liquid crystal material and (2) providing a method for producing the liquid crystal display device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.